This invention relates generally to radio-frequency (RF) power amplifiers and, more particularly, to techniques for combining power control and predistortion in RF power amplifiers used in communication systems. RF power amplifiers are typically the most expensive single components of transmit sub-systems. The cost of the amplifier is proportional to the amount of power that it is required to produce. For this reason, and to improve amplifier efficiency, it is often a design goal to operate each power amplifier as close as possible to its maximum power level. Unfortunately, power amplifiers do not behave in a perfectly linear fashion all the way up to their maximum power levels. That is to say, the amplifier power output is not exactly proportional to the amplifier input for all power levels. Typically, the relationship between output and input power is linear over a range of relatively low power levels but becomes nonlinear at higher power levels. One solution to this problem is to select an amplifier of larger maximum power and to “back off” the amplifier such that it always operates in the linear range. This approach obviously imposes a significant cost penalty.
Predistortion of the amplifier input is a better solution for compensating for nonlinearity of a power amplifier. Basically, predistortion involves applying a nonlinear correction to the amplifier input signals, such that the predistortion correction exactly compensates for the nonlinearity of the amplifier, and the amplifier output is linearly related to the input all the way up to the amplifier's maximum power level. Predistortion is sometimes referred to as “linearization.” Because amplifier nonlinearity affects the phase of the output signal as well as the amplitude, predistortion involves phase correction as well as amplitude correction of the input signal. In other words, predistortion requires a complex multiplication of the input signal by a complex correction factor.
One reason that predistortion or linearization is so important is that power amplifiers used in modern communication systems do not operate at constant power levels. Many of today's complex modulation schemes result in communication waveforms that do not have a constant power envelope. Typically, the signal power varies across a significant dynamic range of several decibels (dB) or more. As a result, the power amplifier must not only be able to transmit a specific amount of power at any instant of time, but it must reproduce the signal with very little nonlinear distortion in order to pass stringent governmental guidelines. Nonlinearity results in unwanted intermodulation products that interfere with adjacent channels. In the United States, for example, the Federal Communications Commission (FCC) has imposed guidelines regarding signal fidelity and adjacent channel power.
A related requirement for transmitter power amplifiers is that a transmitter must typically also control its output power to a very fine degree, even down to tenths of a deciBel (dB). This requirement exists independently of the requirement for linearity. Yet, because predistortion inherently involves amplitude and phase correction, the dual requirements of nonlinearity compensation and fine power control are closely related. A necessary attribute of fine power control is that it must simultaneously co-exist with the predistortion control.
There are numerous methods of predistortion. By way of example, one method of the prior art is shown in U.S. Pat. No. 6,236,837 B1, issued to Pallab Midya. The Midya technique is typical of those in which one has access to a “reference waveform” (i.e., the originally generated baseband waveform before upconversion and amplification). The technique involves detecting the power amplifier output, downconverting the output signal to baseband, and comparing this “transmit waveform” to the reference waveform. The resulting amplitude and phase errors are inverted and fed back to subsequently transmitted signals.
The requirement to have both power control and predistortion successfully co-exist presents several challenges and problems. FIG. 1 is a block diagram of an exemplary adaptive digital predistortion system that represents the current state of the art in predistortion approaches for communication systems. An RF power amplifier 10 generates an output on line 12, a sample of which is obtained through a coupler 13 and fed to a demodulator 14 that separates provides in-phase (I) and quadrature (Q) components of the demodulated output signal, referred to as a transmit burst. These signal components are input to a background processor 16, which also receives corresponding components of a reference burst from a waveform generator 18. The principal task of the background processor 16 is to continually update a predistortion database 20, which contains correction factors to be applied to the input signals. The waveform generator 18 supplies input signals to a complex multiplier 22, the outputs of which are supplied to a transmit chain 24, and from there are input to the power amplifier 10. When the amplifier 10 is operating in its linear range, the complex multiplier 22 has an insignificant effect on the input signals, which are passed through the transmit chain 24 to the power amplifier 10. A real-time processor 26 uses information from the predistortion database 20 to compute an appropriate complex multiplier to be applied to the input signals from the waveform generator 18. The predistortion correction is applied in the complex multiplier 22. Depending on the initial conditions and on the specific control algorithm employed in the real-time processor 26, linearization of the amplifier 10 may take multiple iterations of the control loop described. The amplifier 10 is an analog device but, for reasons of convenience and economy, predistortion is best implemented with digital components. The necessary digital-to-analog and analog-to-digital conversions are omitted from FIG. 1 for simplicity.
Not shown in FIG. 1 is any method of controlling the transmit output power. In a typical multi-channel operating environment, not only must the predistortion system be able to apply the correct amount of nonlinear compensation, but the amplifier must also have some means for linear gain compensation for each channel. Fundamental considerations in the design of a power control sub-system include the ability to compensate for the small signal gain variation across the frequency operating band (from channel to channel) and the ability to ensure stability in the combination of gain correction with predistortion.
Approaches for combining adaptive power control with adaptive digital predistortion have focused on autonomous power control. In autonomous power control, a traditional approach to controlling transmit output power is used in a system employing predistortion. Typically, the power amplifier output is coupled to a calibrated power detector, which feeds a compensation circuit, including such components as RF attenuators, a variable gain amplifier, and/or a digital baseband gain block. One drawback of this approach is that it requires additional hardware, which increases the product cost and complexity. A more subtle complication is the potential for system instability due to the predistortion nonlinear gain correction. A momentary improper power control setting can result in a significantly erroneous error array used for the predistortion update, which in turn can result in an incorrect nonlinear gain setting, which would improperly affect the output power. Another difficulty can arise if, for example, the system temperature changes during a period of system inactivity. Both the power control and predistortion settings would initially be incorrect and would have to independently converge. This convergence at best would require many adaptive iterations, resulting in a period of poor performance, and at worst the system could settle to an improper operating state.
It will be appreciated there is a significant need for an RF power amplifier circuit that effectively integrates amplifier predistortion with fine amplifier gain control. The present invention is directed to this end.